Gas discharged from, for example, a combustor or a reaction device contains impurities to be removed. For example, an oxyfuel combustor has been reviewed as one of techniques for abating emission of carbon dioxide (CO2) which is said to be one of factors for global warming, and attention has been attracted to a coal-fired boiler for oxyfuel combustion of pulverized coal. It has been conceived in the coal-fired boiler that oxygen in lieu of air is used as an oxidizing agent to produce exhaust gas mainly composed of carbon dioxide (CO2) and the exhaust gas with high CO2 concentration is compressed and cooled into liquefied carbon dioxide which is then transported by a vessel, a vehicle or other carrier means to a destination for storage thereof in the ground or alternatively the liquefied carbon dioxide increased in pressure is transported through a pipeline to a destination for storage thereof in the ground.
Such exhaust gas from the coal-fired boiler for oxyfuel combustion contains, in addition to carbon dioxide (CO2), impurities derived from coal feedstock such as nitrogen oxides (NOx), sulfur oxides (SOx), mercury (Hg), hydrogen chloride (HCl) and dust. Such impurities require to be removed since the impurities may cause environmental contamination, corrosion and the like and admixture of the impurities may lower a purity degree of carbon dioxide (CO2) discharged.
Among the above-mentioned impurities, sulfur oxides (SOx) may be contacted with and dissolved in water into sulfuric acid (H2SO4) and hydrogen chloride (HCl) may be dissolved in water into hydrochloric acid, so that such water-soluble sulfur oxides and hydrogen chloride as well as dust may be separated through contact with water by, for example, water spraying.
Among the nitrogen oxides (NOx) as the above-mentioned impurities, nitrogen dioxide (NO2) may be contacted with and dissolved in water into nitric acid (HNO3) to become separated. However, the exhaust gas from the coal-fired boiler has less oxygen (O2) so that nitrogen (N2) exists substantially in the form of nitrogen monoxide (NO) which is water-insoluble and thus is unremovable by, for example, water spraying.
Among the above-mentioned sulfuric, hydrochloric and nitric acids, especially sulfuric acid is known to corrode instruments in an exhaust gas treatment device; mercury, which is trace metal, is known to hurt low-temperature aluminum members constituting a heat exchanger. Thus, it is preferable to remove these impurities at early stages. There is also a problem that admixture of the impurities into the exhaust gas lowers a purity degree of the carbon dioxide, which makes troublesome the liquefaction of the carbon dioxide through compression and cooling and thus requires large-sized equipment therefor. Thus, in a coal-fired boiler for oxyfuel combustion or other system where produced is exhaust gas mainly composed of carbon dioxide which in turn is to be disposed, it is extremely important to remove the impurities in the exhaust gas.
Thus, it has been conducted, for example, in the coal-fired boiler for oxyfuel combustion that a spray-column-type, packed-column-type or other so-called wet desulfurizer used in a conventional air-fired boiler or the like is provided to remove sulfur oxides which are especially problematic in corrosivity. Moreover, nitrogen and nitrogen oxides derived from coal feedstock are produced in the exhaust gas from coal-fired boiler for oxyfuel combustion or the like, so that it has been conducted that a catalyst-type or other denitrator is arranged upstream of the desulfurizer to remove nitrogen and nitrogen oxides.
It is known that the provision of the wet desulfurizer as mentioned in the above removes sulfur oxides, hydrogen chloride and dust as well as part of nitrogen oxides and slightly removes mercury, which is inherently low in content. It has been also conceived that if mercury in the exhaust gas is still high in concentration even after the above-mentioned exhaust gas treatment is conducted, a mercury-removing column is arranged to remove the mercury by adsorbent or the like.
An exhaust gas treatment system comprises, for example, a duct with a dust collector and a wet desulfurizer for guidance of exhaust gas from a boiler which in turn burns fuel with combustion gas in the form of a mixture of oxygen-rich gas with circulation exhaust gas, an exhaust gas recirculation duct for guidance of part of the exhaust gas downstream of the dust collector to the boiler and CO2 separation means for compression of the exhaust gas downstream of the desulfurizer to separate carbon dioxide, water separated during the compression of the exhaust gas by the CO2 separation means being supplied to absorbing liquid used circulatorily in the desulfurizer (see Patent Literature 1).
Patent Literature 2 discloses an exhaust gas treatment system for an oxyfuel combustor with a front impurity removal device and at least one rear impurity removal device. The front impurity removal device comprises a compressor for compression of exhaust gas from the oxyfuel combustor to make water-soluble the impurities in the exhaust gas and a cooler for cooling of the exhaust gas compressed by the compressor to condense water and discharge drain with the impurities dissolved. The or each rear impurity removal device comprises a rear compressor for compression of the exhaust gas at a pressure higher than that in the compressor and a rear cooler and serves for discharging drain.
Patent Literature 3 discloses a carbon dioxide purification device comprising a compressor for elevation in pressure of gaseous carbon dioxide; at least one countercurrent gas/liquid contact device for washing of the gaseous carbon dioxide with water at elevated pressure in the presence of molecular oxygen and, when SO2 is to be removed, NOx for a sufficient time to convert SO2 to sulfuric acid and/or NOx to nitric acid; conduit means for feeding of the gaseous carbon dioxide at elevated pressure from the compressor to the or each gas/liquid contact device; and conduit means for recycling of aqueous sulfuric acid solution and/or aqueous nitric acid solution to the or each gas/liquid contact device.
Patent Literature 4 discloses a device for simultaneous treatment of dust collection and desulfurization wherein ash-containing boiler exhaust gas is cooled to or less than 40° C. to condense water in the exhaust gas, using boiler feeding water or boiler combustion air, exhaust gas at an outlet of a desulfurizing absorbing column or one or more kinds of seawater; SOx in the exhaust gas is removed, using desulfurizing absorption liquid which is a slurry of ash and condensed water admixed with lime; and unrequisite ash is separated by an unrequisite ash sedimentation/separation device below an absorption column tank.
Patent Literature 5 discloses a combustion exhaust gas purification system comprising a first process for making gas/liquid countercurrent contact of combustion exhaust gas with an aqueous alkali metal carbonate solution containing at least 0.1 N of alkali metal carbonate, using a leaking tray column, to reduce sulfur oxides and nitrogen oxides in the combustion exhaust gas; a second process for making gas/liquid countercurrent contact of the combustion exhaust gas from the first process containing carbon-rich gas and nitrogen with an aqueous alkali metal hydroxide solution, using a leaking tray column, to convert at least part of the carbon-rich gas in the exhaust gas into alkali metal carbonate to thereby purify the carbon-rich gas; and a regeneration process of the aqueous alkali metal hydroxide solution for reacting the alkali metal carbonate produced as a by-product in the second process with alkaline earth metal hydroxide to produce and separate alkaline earth metal carbonate to thereby withdraw the aqueous alkali metal hydroxide solution.